Conveyor belt system with positional transformation of weight data

ABSTRACT

A scale position compensation means for a conveyor belt material transport system, the system including a conveyor belt having a tail end and a discharge end, belt drive means, and a weight signal generating means. Belt travel signal generating means produce a belt travel signal representing the change of belt position. The weight signal generating means produce a first digital weight signal representing a weight of material associated with a portion of the belt adjacent to a first reference point located between the tail and discharge ends. The scale position compensation means are responsive to the first digital weight signal and the belt travel signal to produce a second digital weight signal representing a weight of material associated with the abovementioned portion of the belt when it is adjacent to a second reference point located between the first reference point and the discharge end. The compensation means comprise a memory for storing the first digital weight signal for a time interval having a duration related to the rate of change of belt position and to the distance between the first and second reference points.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 599,315 filed July 25,1975, abandoned; which is a divisional application of Ser. No. 495,068filed Aug. 5, 1974, now U.S. Pat. No. 3,960,225; which is a continuationof application Ser. No. 418,088, filed Nov. 21, 1973, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates generally to material transport systemsand more particularly to conveyor belt systems.

In many conveyor belt applications it is desirable to measure or controlthe weight of material moving past a first reference point on a belt,which material is delivered downstream to a second reference point onthe belt, usually the discharge end. Consequently, substantial efforthas been expended to develop accurate conveyor belt scales which utilizean input sensor at the first reference point for producing an outputsignal related to the weight of material on an associated portion of thebelt. Examples of such scales are the Thayer Series RF Belt Scalesmanufactured by Hyer Industries, Inc., the assignee of the presentinvention. Scales of this type permit in-motion weighing for weighttotalizing and flow rate control in material transport systems. In manyapplications such scales are used in a system incorporating anelectronic integrator which receives weight signals from a belt scaleand a belt speed signal from the conveyor belt drive means. Theintegrator intregrates the product of these two signals and provides anoutput signal which is indicative of the weight of material that passeson the portion of the belt associated with the scale input sensor.Electronic integrators of this type are well known in the art, and maybe of the form shown in U.S. Pat. No. 3,610,908 to Raymond Karosas,dated October 5, 1971 and assigned to the assignee of the presentinvention.

U.S. Pat. No. 3,559,451 to Frank S. Hyer and Raymond Karosas, dated Feb.2, 1971, and assigned to the assignee of the present invention,describes a totalizing and flow rate measuring system which includes anintegrator of the type noted above to generate a digital weight signalwhich is subsequently processed to produce output signals or indicationsrepresentative of the cumulative weight and the instantaneous flow rateof material on the belt which passes the input sensor of the scale.

In the conveyor belt material transport systems known in the art, asdescribed above, a substantial problem may arise from the choice of thebelt scale input sensor location between the tail and discharge ends ofthe belt. This problem results from the conflicting requirements ofin-motion weight measurement on the one hand, and of providing accuratedata on material delivery or correlation with external processes on theother hand. From the standpoint of in-motion weight measurement, theoptimum location of the scale input sensor for accurate measurement isat the point of least belt tension in the conveyor, allowing for thematerial to settle in stable form on the belt prior to reaching thescale input sensor. This requirement dictates the placement of the scaleinput sensor near the tail end of the conveyor belt. With the scaleinput sensor so located, any cumulative weight measurement or flow ratemeasurement is related to a position remote from the discharge end.Therefore, such a system does not reflect the quantity of material onthe belt portion between the sensor and the discharge end or variationsin the material loading along that portion of the belt. The resultinginaccuracies are referred to herein as problems of transport lag. Thematerial being delivered by the conveyor transport system to an externalprocess or to a receiving container (e.g., a truck or railroad car) isthat which is discharged from the head pulley or discharge end.Therefore, from the standpoint of accuracy in material delivery orexternal process, the optimal position for the scale input sensor wouldbe at the discharge end. However, in certain applications, when theinput sensor of a belt scale is positioned near the relatively hightension discharge end of a conveyor belt, the accuracy of the scale issubstantially impaired.

In the prior art, the usual practice has been to position the scaleinput sensor near the lower tension tail end whenever the input weighingaccuracy is of primary importance (e.g., plus or minus one-half percentor better), to position the scale near the higher tension discharge end(and bearing with the resulting scale inaccuracies) when the outputweighing accuracy is of primary importance, and to position the scale atsome intermediate point selected to compromise between the conflictingrequirements when they are of more nearly equal importance. Inimplementing such a trade-off, the loss in performance accuracy due toboth effects is a substantial drawback in certain prior art conveyorbelt material transport systems.

A further difficulty arises in certain prior art systems where theconfiguration of the conveyor belt does not permit the positioning of ascale input sensor in an appropriate position relative to the belt. Insuch systems a weigh feeder, such as the Thayer Series MXL weigh feedermanufactured by Hyer Industries, Inc., may be used to deliver thematerial to the conveyor belt. A weigh feeder typically comprises a beltscale and a relatively short conveyor belt having substantially none ofthe scale accuracy and transport lag problems associated with therelatively long main conveyor belt. The drive means of the weigh feederbelt are driven by a demand weight signal which represents the desiredweight of material to be fed to the main belt from the discharge end ofthe weigh feeder belt. However, even when the weight of material addedto the main belt in this manner is accurately measured and controlled bythe weigh feeder, the system may have inaccuracies due to transport lagas will be clear from the following description. These difficulties area consequence of the inability to identify the precise time at whicheach portion of material added by the weigh feeder has reached aparticular downstream point on the main belt.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a conveyorbelt material transport system having means for generating a signalrepresentative of a weight of material associated with a portion of thebelt adjacent to a first reference point between the tail and dischargeends of the belt, and having a means for generating a signalrepresentative of a weight of material associated with that portion ofthe belt when it is adjacent to a second or downstream reference point.The first-mentioned signal may be generated by actually measuring theweight of material on the belt, as by a belt scale, or by generating asignal independently of the weight of material on the belt, as by asource of demand pulses representing the weight of material to be placedon the belt.

In accordance with the present invention, a conveyor belt materialtransport system is provided with a scale position compensating means.The conveyor belt transport system includes a conveyor belt having atail end, a discharge end, a belt drive means and a weight signalgenerating means. The weight signal generating means produces a firstdigital weight signal representative of a weight of the materialassociated with a portion of the belt adjacent to a first referencepoint located between the tail and discharge ends. The system alsoincludes a belt travel signal generating means which produces a belttravel signal representative of the change of position of the belt.

The scale position compensation means are responsive to the firstdigital weight signal and the belt travel signal to produce a seconddigital weight signal representing a weight of material associated withthe above-mentioned portion of the belt when it is adjacent to a secondreference point located between the first reference point and thedischarge end. The compensation means comprise a memory for storing thefirst digital weight signal for a time interval having a durationrelated to the rate of change of belt position and to the distancebetween the first and second reference points.

In one embodiment the scale position compensator comprises a multiplestage shift register. In this embodiment, the belt travel pulse a signalis in the form of a sequence of pulses, wherein each pulse represents apredetermined distance of belt travel. The belt travel signal is used asthe shift signal for the shift register and is effective to shift thefirst digital weight signal through the multiple stage register. Thenumber of stages, D, required for the shift register is defined by theequation:

    D = NL,

where N is the number of pulses generated by the belt travel pulsegenerator for each foot of belt travel and L is the distance in feetbetween the first reference point and the downstream second referencepoint.

The output signal of the last stage of the shift register is the seconddigital weight signal and is thus a delayed representation of the firstdigital weight signal. The delay is precisely matched with the distancebetween the reference points so that the shift register output signalrepresents a weight of material associated with the same portion of thebelt that was represented by the first digital weight signal, but isproduced when that portion of the belt is adjacent to the secondreference point.

The weight signal generating means may comprise a belt scale having aninput sensor associated with the portion of the belt located at thefirst reference point, and a digital flow integrator means responsive tothe sensor output weight signal and to the belt travel signal to producethe first digital weight signal. The scale sensor may be positioned nearthe low tension tail end of the conveyor belt. The second digital weightsignal produced at any given moment by the scale position compensationmeans is representative of the weight of material then located at thedischarge end of the conveyor belt (i.e. the second reference point)since that signal is a delayed form of the first digital weight signaland the delay is precisely matched with the belt motion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conveyor belt material transport system embodying thepresent invention.

FIG. 2 shows an alternative form of the system of FIG. 1.

FIG. 3 shows a control sub-system for use with the system of FIG. 2.

FIG. 4 shows another alternative embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an embodiment of a conveyor belt material transport systemin accordance with the present invention. A conveyor belt 12 has a tailend 14, a discharge end 16, and a belt drive means or motor 18 at thedischarge end. The drive motor 18 may comprise, for example, a d.c.motor connected to the discharge end pulley 20. A second pulley 22 islocated at the tail end 14 of the belt. In operation, the belt drivemeans 18 drives the pulley 20 which, in turn, produces a motion in thebelt 12 so that material deposited on the top surface of the belt 12 istransported in a direction toward the discharge end 16.

A weight signal generator 30 provides a first digital weight signal on aline 30a which is representative of a weight of material associated witha portion of the belt 12 adjacent to a first reference point 31 locatedbetween the tail and discharge ends 14 and 16 of the belt. The firstdigital weight signal may represent the weight information in any one ofseveral digital forms in accordance with known techniques. It is appliedby way of a line 30a to a first input terminal of scale positioncompensation means 40.

The scale position compensation means 40 includes a memory meanscomprising a multiple stage shift register 42 which is adapted to shiftthe first digital weight signal applied via the line 30a in response tobelt travel pulses generated by a belt travel signal generator 36 andapplied via a line 36a. The last stage of the shift register 42 producesa second digital weight signal on a line 40a, the second digital weightsignal being representative of a weight of material associated with thesame portion of the belt 12 when that portion is adjacent to a secondreference point 32.

The belt travel pulse generator 36 may be coupled to the tail end pulley22, and produces an output belt travel signal comprising a sequence ofpulses on the line 36a. The belt travel signal pulses are thus generatedby movement of the belt. Alternatively, they may be generated by thepower frequency if the drive means comprise a synchronous motor. Therepetition rate of the pulses on the line 36a is directly proportionalin either case to the belt velocity past the pulley 22. For example, thegenerator 36 may generate one hundred pulses per foot of belt travel, inwhich case the number of pulses generated per second by the generator 36is one hundred time the belt speed in feet per second. The line 36a isconnected as a second input to the scale position compensator 40.

In the embodiment of FIG. 1 the first digital weight signal representsthe momentary weight of material on a portion of the belt adjacent tothe reference point 31, and it is this information which passes throughthe scale position compensator 40. FIG. 2 shows a particular embodimentin which the information passing through the scale position compensatoris in the form of pulses each representing a specific weight ofmaterial. A loading hopper 24 is positioned near the tail end 14 fordepositing material 21 on the top surface of the belt 12. The firstreference point 31 is positioned near the tail end 14 but downstreamfrom the hopper 24. The second reference point 32 is positioned at thedischarge end 16 of the belt. It will be understood that elements inFIG. 2 which are the same as corresponding elements in FIG. 1 aredenoted by the same reference numerals.

The weight signal generator 30 in FIG. 2 includes an inmotion belt scaleinput sensor 52 and a digital flow integrator 54. The sensor 52 isassociated with a portion of the belt 12 adjacent to the first referencepoint 31. The effective center line passing through the input sensor 52and perpendicular to the top surface of the belt 12 is located apredetermined distance "L" from the second reference point 32 located ona center line of the pulley 20 which passes through the rotational axisof the pulley and perpendicular to the top surface of the belt.

The scale input sensor 52 may be of the Thayer Series RF Belt Scale typemanufactured by Hyer Industries, Inc. Such scale systems comprise anaccurate weighbridge constructed of steel to provide rigidity and ofminimum surface area on which material can accumulate. The Series RFscales further include means for mass counterbalancing of dead loadingsuch as scale parts, belting weight and scale idlers so that the scaleinput sensor produces a d.c. output signal proportional in amplitude tothe net weight of the material on the portion of belt 12 associatedtherewith.

The scale 52 produces an analog weight signal on a line 52a which signalmay be, for example, a d.c. voltage having an amplitude proportional tothe weight of material 21 per unit length of the belt 12. The analogweight signal on the line 52a is applied to a first input terminal ofthe integrator 54.

The integrator 54 also receives a belt travel signal comprising asequence of pulses having a repetition rate proportional to the velocityof the belt, each pulse representing a predetermined distance of belttravel.

The integrator 54 produces a first digital weight signal on the line30a. This signal is generated from the integration of the product of theanalog weight signal and the belt travel signal. From the resultantsignal a sequence of pulses is generated on the line 30a having arepetition rate proportional to the mass flow rate of the materialpassing the scale sensor 52. Thus the integrator 54 generates a firstdigital weight signal wherein each component pulse represents apredetermined unit of weight of material 21 on the belt 12 which haspassed the scale input sensor 52. Such integrators are well known in theart. For example, the integrator may comprise a Thayer I-128 DigitalFlow Integrator manufactured by Hyer Industries, Inc.

The first digital weight signal pulses from the integrator 54 may beused for a number of purposes. For example, they may be applied to atotalizing counter (not shown) which produces an output signal orindication of the cumulative weight of the material passing the scaleinput sensor 52. Alternatively, the first digital weight signal may betransformed to a time rate signal representative of the amount oftonnage (or other weight unit) of material being transported per hourpast the scale input sensor 52. In some embodiments the first digitalweight signal on the line 30a may be compared with an independentlyproduced reference flow signal to a "demand" control system. Suchsystems are well known in the art and are not shown in FIG. 2 to avoidcomplication of the drawing. Briefly, the comparison produces an ouputcontrol signal for a controller modifying the signal to the belt drivemeans 18. The control signal is based on an error signal between thereference or demand signal and the first digital weight signal andcompensates for the error. In this manner a closed loop demand feedsystem may be provided which assures a predetermined rate of flow pastthe scale input sensor 52.

The material transport system thus far described with reference to FIG.2 is well known in the art. As noted above, the position of the scaleinput sensor 52 has created a substantial problem in certain instances.The usual practice has been to achieve a trade-off of the transport lagand the scale accuracy considerations by positioning the scale inputsensor somewhere between the lower tension tail end 14 where scaleweighing accuracy would be greatest and the higher tension discharge end16 where transport lag would be eliminated. By the addition of the scaleposition compensation means in accordance with the present invention,this scale positioning trade-off is obviated. The compensation means 40comprise a memory element which may be in the form of a multiple stagedigital shift register 42, as in FIG. 1. For example, in FIGS. 1 and 2the register 42 comprises a series of type D flip-flops with the shiftinput connected to the line 36a and the data input connected to the line30a. The first digital weight signal from the line 30a is applied to thefirst shift register stage SR-1, and is shifted from stage to stagethrough the register 42 by the digital belt travel signal produced onits line 36a by the generator 36. A second digital weight signal isapplied on the line 40a from the last shift register stage SR-D. It willbe understood that the pulse repetition rate of the shift signal is atleast twice that of the first digital weight signal so that noinformation in the latter signal is lost during the storage operation.

In the embodiment of FIG. 2 the first digital sensor weight signal fromthe line 30a, representing unit weight increments of the material 21passing over the input sensor 52, is stored in the compensator 40 for atime interval equal to the transit lag in the delivery of the materialfrom the reference point 31 adjacent to the efective center line of thescale input sensor 52 to the reference point 32 at the discharge end 16of the conveyor belt 12. By utilizing a shift register for the memorymeans, and shifting the first digital weight signal through thatregister by means of the belt travel pulses, a precise delay is achievedto compensate exactly for the position of the input scale sensor 52relative to the reference point 32. Consequently, each pulse of theshift register 42 output, i.e. the second digital weight signal on line40a, is representative of a unit weight increment of material 21 passingthe reference point 32. The number of shift register stages "D" isdetermined in the same manner described above with reference to FIG. 1.

In FIG. 2 the belt travel signal generator 36 is coupled directly to thepulley 22. The generator 36 may be a rotary shaft position encoderconnected to the shaft of the pulley 22. A suitable encoder is a Type Trotary pulse generater manufactured by Trump-Ross Industrial Controls,Inc. Alternatively, as stated above, the belt drive motor 18 may besynchronized to the power line frequency and thus operate at a constantspeed. In the latter alternative the belt speed is proportional to thefrequency of the power line and the belt travel pulse generator 36 maybe connected to the power line to generate a clock signal at a frequencyproportional to the power line frequency. Thus the generator 36 may ormay not have a connection to the tail end pulley 22 of the belt or otherparts of the belt drive mechanism. The number of shift register stagesrequired in the scale position compensator 40 is in any case a functionof the repetition rate of the clock signal generated by the pulsegenerator 36 and the distance L. If the transport system is such thatthebelt may move at other than a constant speed, then the number ofshift register stages is determined by providing a belt travel signalpulse for each unit distance of belt travel and computing the number ofunits of travel in the length L.

As an alternative to the use of the integrator 54, the weight signalgenerator may include only a scale input sensor and the output of thissensor may comprise the first digital weight signal. In such case, thesecond digital weight signal produced on the line 40a may then beapplied to an integrator of the type described above. This integratormay also be operable by a belt travel signal to produce a third digitalweight signal comprising pulses of the type described above, and thesepulses will relate to unit weight increments at the point 32.

FIG. 3 shows an exemplary control sub-system 60 for connection with theconveyor belt system of FIG. 2. The sub-system 60 includes an inputconnection to the line 40a from the scale position compensator 40 ofFIG. 2, and an output connection 60a to the belt drive motor 18 of FIG.2. The control system 60 includes a totalizer 61, a digital controller62, a predetermined counter 65 and a rate detector 66, all connected tothe input line 40a.

The digital controller 62 includes an up-down counter 63 having itsoutput connected to a digital-to-analog converter 64. The line 40a isconnected to a count-down input of the up-down counter 63. The counter63 has a second, count-up input connection on a line 68a from a demandgenerator 68. The ouput of the converter 64 is applied to a motorcontrol means 69 having its output applied by a line 60a to the beltdrive motor 18 to control its speed.

The line 40a is connected to the count input of the predeterminedcounter 65. The preset input of the counter 65 is connected to a presetcontrol 67 via a line 67a. The output of the counter 65 is connected byway of the control 69 and the line 60a to the belt drive motor 18.

In operation, as described in conjunction with FIG. 2, the seconddigital weight signal on the line 40a from the last shift register stageof compensator 40 is a series of pulses, each pulse having entered thefirst stage representing a unit weight of material 21 on the portion ofthe belt 12 adjacent to the scale sensor 52. Since the shift registercompensating means 40 has the appropriate number of stages toaccommodate the transport lag resulting from the belt movement over thedistance L between the first reference point 31 and the second referencepoint 32, the totalizer 61 provides a measure of the cumulative weightwhich has been delivered at the discharge end 16 of the belt 12.

The compensator 40 output pulse sequence on the line 40a is also appliedto the rate detector 66. This detector is effective to provide an outputsignal representative of the repetition rate of the pulses in the seconddigital weight signal on the line 40a, and is therefore a measure of theflow rate of the material 21 passing the discharge end 16 of the belt12.

The digital controller 62 and the predetermined counter 65 permit thematerial transport system to perform in either or both of two modes:first, delivery of a predetermined total weight of material 21 at thedischarge end 16 and, second, delivery of the material 21 at apredetermined rate at the discharge end 16.

In the first mode the counter 65 is utilized. This counter is preset toa count state corresponding to the desired number of weight units ofmaterial 21 to be delivered. This is accomplished by an appropriatesignal on the line 67a. As material is delivered to the discharge end16, as indicated by pulses on the line 40a, the counter 65 counts downtoward the zero state from the preset state. When the counter 65 reachesthe zero state, the desired amount of material 21 has been delivered atthe discharge end 16. The motor control 69 includes logic circuitry of atype already known in the art and effective to determine when thecounter 65 is in a non-zero state, to signal this state by means of acounter 65 output signal on a line 65a, and to generate an appropriatesignal on the line 60a to energize the motor 18. When the counter 65reaches the zero state the control 69 applies a disabling signal to themotor 18. Thus the control 69 provides a "go-on go" control. In thismode of operation, the counter 65 is preset to indicate the demandedamount of the material 21 to be delivered, and the belt drive motor 18is driven in response thereto so that the belt 12 continuously deliversmaterial to the discharge end 16 until the counter 65 has counted downin decrements to its zero state, indicating that the demanded amount ofmaterial 21 has been delivered at the discharge end 16.

In the second mode of operation of the system of FIGS. 2 and 3, thedigital controller 62 is utilized. The demand generator 68 applies asequence of pulses via the line 68a to the count-up input of counter 63at a repetition rate corresponding to a desired flow rate at the secondreference point 32. These demand pulses advance the counter 63 inincrements upward from its zero state. As material 21 is delivered tothe discharge end 16, the scale position compensator 40 applies thesecond digital weight signal to the count-down input of the counter 63.The converter 64 includes appropriate circuitry to transform the outputsignal from the counter 63 to an "error" analog signal on the line 64a,the amplitude of the analog signal being related to the count state ofthe counter 63. The convertor output signal is applied to the control 69which in turn provides a driving signal for the motor 18. The motor 18drives the belt 12 at a speed which is related to the "error" signalproduced by the counter 63, this signal speeding up or slowing down themotor 18 as may be necessary to cause the material to be delivered atthe demand rate. Thus the input demand rate is exactly matched by thedischarge rate and a predetermined flow rate at the discharge end 16 ofthe belt is achieved.

Referring to FIG. 1, a weigh feeder may be used for depositing materialon the belt at a predetermined rate and at any designated feed pointthereon. This may be accomplished by means of a weigh feeder demandsignal and a weigh feeder such as the Thayer Series MXL manufactured bythe assignee of the present invention. The weigh feeder demand signalmay comprise a sequence of pulses each representing a weight of materialto be deposited at the feed point. These pulses may be delivered to adigital controller like the unit 62 in FIG. 3. The weigh feeder may beprovided with a scale sensor, the output signal from which is alsodelivered to the digital controller. The digital controller may controlthe speed of the weigh feeder conveyor belt, and therefore the rate ofmaterial delivered to the feed point, in the same way that the motor 18is controlled in the embodiment of FIG. 3. This arrangement may beutilized, for example, when blending two materials X and Y separatelydeposited at different feed points on the belt, the blending beingaccomplished in a predetermined weight proportion. Two specific multiplefeed point embodiments are described below.

First, with reference to FIG. 1, assume that a wild flow of the materialX, that is a flow occurring at a nonconstant rate, is added to the belt12 at a point upstream from the first reference point 31, and furtherthat a weigh feeder (not shown) is used to deposit the material Y at thesecond reference point 32. In this case the weight signal generator 30associated with the point 31 includes a scale input sensor, anintegrator and ratio control means for generating a first digital weightsignal on the line 30a. This first digital weight signal isrepresentative of the weight of the material Y to be added to the belt12 as required to achieve the predetermined proportionally of thematerials X and Y. For example, assume that the system has a digitalweight signal comprising sequences of pulses wherein each pulserepresents one pound of the associated material, and further that thepredetermined weight ratio of materials X to Y is two to one. In thiscase, through operation of the ratio control means, the generator 30applies a single pulse on the line 30a for each two pounds of material Xdetected at the point 31 on the belt 12 by the scale sensor. Thus thefirst digital signal is derived from a measurement of the weight of thematerial X at the point 31. This signal is applied to the data input ofthe shift register 42. The belt speed travel signal on the line 36a iseffective to shift the first digital weight signal through the register42, to generate the second digital weight signal on the line 40a. Thislatter signal serves as the demand signal for the weigh feederassociated with the point 32. The first digital weight signal is delayedfor a time interval having the precise duration to permit the materialat the first reference point 31 to be transported to the secondreference point 32. In response to the second digital weight signalapplied as the weigh feeder demand signal, the weigh feeder deposits thecorresponding weight of material Y on to the belt 12 at the referencepoint 32 at the precise time to achieve the predetermined proportion ofthe materials X and Y at the point 32 and at all points downstreamthereof. Thus compensation is provided for the position of the scalesensor associated with the point 31 relative to the second feed point 32in a manner permitting in-phase proportioning to a wild flow of thematerial X on the belt.

A second mutliple feed point embodiment is shown in FIG. 4. Thiscomprises a single belt blending system having two weigh feedersconstructed and controlled as described above and positioned along thebelt for delivering materials X and Y at the feed points 31 and 32,respectively, with a predetermined proportionate weight. The elements ofthe system of FIG. 4 which correspond to similar elements of FIG. 1 areidentified with the same reference numerals. In the system of FIG. 4 theweight signal generator 30 comprises a demand generator which provides afirst digital weight signal on the line 30a. This signal comprises asequence of pulses wherein each pulse is representative of apredetermined composite weight of the materials X and Y which is desiredto be delivered to the second reference point 32 and all pointsdownstream thereof. The first digital signal is applied by way of aratio control 72 to a weigh feeder 74 for discharging the material X atthe point 31. The ratio control 72 performs an appropriate frequencydivision of the first digital signal to provide a weigh feeder demandsignal, in response to which the weigh feeder 74 deposits thepredetermined proportion of the material X at the point 31.

The first digital signal on the line 30a is also applied to the datainput of the scale position compensation means 40. The belt travelsignal generator 36 applies a belt travel signal via the line 36a to theshift input of the compensation means 40. The compensation means 40 maycomprise a multiple stage shift register and functions in the samemanner as the register 42 described above in conjunction with FIG. 1.Accordingly, the shift register has the number of stages required tocompensate for the transport lag encountered over the distance, L,between the points 31 and 32.

The second digital weight signal on the line 40a is applied by way of aratio control 76 to a weigh feeder 78 whereby material Y is dischargedtherefrom at the point 32. The ratio control 76 performs an appropriatefrequency division of the second digital signal to provide a weighfeeder demand signal, in response to which the weigh feeder 78 depositsthe predetermined proportion of the material Y at the point 32. In thismanner, the predetermined proportionate mixture of the materials X and Yis provided on the belt 12 at the point 32 and all points downstreamthereof.

It will be understood that in this latter embodiment, the weight signalgenerator provides a first digital weight signal which corresponds tothe composite weight of material to be deposited on the belt 12. Thatis, it is a demand signal that is not produced by the weight of materialactually on the belt. The first digital weight signal is, as described,representative of the weight of material associated with the portion ofthe belt adjacent to the point 31 through the predeterminedproportionally of the composite mixture of materials X and Y.

It will be further noted that this latter system accommodates start-upand shut-down of the demand generator in the generator 30 by maintainingthe desired proportionally of materials X and Y during these periods.For example, during start-up the weight feeder demand signal for thematerial X is applied immediately to the weigh feeder 74 while thecorresponding signal for the weigh feeder 78 is delayed for the precisetime interval required to permit the initial deposits of the material Xto reach the point 32.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentinvention embodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

We claim:
 1. Apparatus for a belt conveying a stream of material thereonhaving, in combination,means to generate, at a time when a portion ofthe belt is adjacent a first reference point, a first characteristicsignal related to weight of material associated with said portion, meansto generate pulses recurrent upon each unit distance of belt travel, andmeans responsive to said signal and to the belt travel pulses togenerate a stream of substantially uniform pulses, wherein each pulse insaid stream represents a unit of weight of material associated with aportion of the belt adjacent a second reference point at the time ofthat pulse, said second reference point being downstream of said firstreference point, wherein the means responsive to said signal comprise amemory for storing the first characteristic signal for a time intervalrelated to the distance between said first and second reference pointsand the rate of belt travel.
 2. Apparatus according to claim 1, in whichthe memory comprises a multiple stage shift register having a data inputfor receiving said first characteristic signal and a shift input forreceiving said belt travel pulses, the number of stages being related tothe number of belt travel pulses per unit distance traveled by the beltand the distance in said units between said first and second referencepoints.
 3. Apparatus according to claim 2, in which the number of stagesequals the product of said distance and said number of belt travelpulses per unit distance.
 4. Apparatus for a belt conveying a stream ofmaterial thereon having, in combination,means to generate, at a timewhen a portion of the belt is adjacent a first reference point, a firstcharacteristic signal related to weight of material associated with saidportion, means to generate pulses recurrent upon each unit distance ofbelt travel, and means responsive to said signal and to the belt travelpulses to generate a stream of substantially uniform pulses, whereineach pulse in said stream represents a unit of weight of materialassociated with a portion of the belt adjacent a second reference pointat the time of that pulse, said second reference point being downstreamof said first reference point, wherein the means to generate the firstsignal includes a demand generator and means operable thereby to varythe weight rate of flow of material on the belt adjacent the firstreference point.
 5. Apparatus according to claim 4 where in the meansresponsive to said signal comprises a memory for storing the firstcharacteristic signal for a time interval related to the distancebetween said first and second reference points and the rate of belttravel.
 6. Apparatus according to claim 5 in which the memory comprisesa multiple stage shift register having a data input for receiving saidfirst characteristic signal and a shift input for receiving said belttravel pulses, the number of stages being related to the number of belttravel pulses per unit distance traveled by the belt and the distance insaid units between said first and second reference points.
 7. Apparatusaccording to claim 6 in which the number of stages equals the product ofsaid distance and said number of belt travel pulses per unit distance.